1. Field of the Invention
The present invention relates to a process for nondestructive testing of test pieces for thermal damage. More particularly, the invention relates to a nondestructive test process for detecting grinding-induced and related thermal damage to chromium-plated metal workpieces in which an electrochemical etching technique is used to cause indications of damaged areas to appear on the test piece surface as color anomalies.
2. Description of the Prior Art
The prior art shows a number of examples of nondestructive test methods such as magnetic particle, radiography, eddy current, ultrasonic, liquid crystal, and fluorescent penetrant; however, only the fluorescent penetrant and magnetic particle inspection processes lend themselves to relatively low cost per unit part on high rates of production. The value of these processes are degraded because they are limited to the detection of relatively gross flaws or discontinuities which have openings to the surface of the test parts.
It is also a requirement in the inspection of certain parts that the presence of structural or metallurgical damage to the base metal and chromium be revealed by the inspection method. Prior art methods, such as radiography, used for detecting subsurface defects and damage suffer from the drawbacks of being excessively tedious and of requiring cumbersome equipment, or they are expensive to use and are not readily adaptable to high-volume test scheduling. Radiography is limited also in that it only detects voids. In addition, because of limitations inherent in its resolving power, radiography is restricted in its ability to "see" defects which are small relative to the thickness of the test piece and the method is also restricted by considerations of part geometry.
It has been found, with respect to eddy current and ultrasonic testing, that the methods are substantially ineffective in chromium plate due to the extraneous indications produced by current density induced residual stress variations. The utility of these prior art eddy current and ultrasonic resting techniques is degraded also in that they are tedious and require interpretation of the test results.
A further limitation of the listed prior art methods is that they are all dependent on surface defects such as cracks and the like to produce indications of damage and they are largely ineffective with respect to the detection of structural alterations or thermally residual stress changes.
In the prior art, J. R. Alburger (U.S. Pat. No. 3,530,045) discloses a nondestructive testing method for electrically conductive metallic coated and plated parts for the detection of surface and subsurface defects on a high rate of production basis. In the Alburger method, the workpiece is immersed either anodically or cathodically in an electrolyte solution of an electrochemical system having an electrode of opposite polarity and an electric current is passed through the electrolyte and the workpiece and other electrode: the electrolyte solution is such that, either a coating is plated out of the solution onto the surface of the workpiece, the thickness of the coating being dependent upon the local current density, or, the p.sup.H of a p.sup.H -sensitive liquid is altered locally near the surface due to current density variations resulting from surface or subsurface discontinuities to thus produce color changes. Surface or subsurface defects in the workpiece cause a variation in current density which results also in variations in the thickness of the coating applied on the workpiece. When the workpiece is removed from the electrolyte solution, the variations in coating thickness produce variations in the coating which are detectable visually to thereby also indicate surface or subsurface defects.
It will be seen therefore, that Alburger's process relies on current density effects on either a p.sup.H -sensitive coating or on a deposited layer. Moreover, it will also be recognized that the process of Alburger is critically dependent on the composition of the electrolytic solution and its successful utilization is largely keyed to the presence of defects which act as physical discontinuities or breaks in the workpiece surface or subsurface. Also, unlike the subject process in which an etching or removal of surface material occurs, Alburger plates a coating of material on the workpiece surface, which coating provides the color variations that are used to indicate surface and subsurface defects.